Electron induced deposition of organic coatings



A. H. TURNER 3,462,292

ELECTRON INDUCED DEPOSITION OF ORGANIC COATINGS Aug. 19, 1969 4 Sheets-Sheet 1 Filed Jan. 4. 1966 4445 H. TURNER INVENTOR.

QM Q42 5 ELECTRON mnucnn DEPOSITION OF ORGANIC commas Filed Jan. 4. 1966 A. H. TURNER Aug. 19, 1969 4 Sheets-Sheet 2 Alli-7V TU/Q/VfR INVENTOR.

147' TO/QA/EYS g- 19, 1969 A'- H. TURNER 3,462,292

ELECTRON INDUCED DEPOSITION OF ORGANIC COATINGS Filed Jan. 4. 1966 4 Sheets-Sheet 5 41 (67V fl. TURA/'R INVENTOR.

Aug. 19, 1969 A. H. TURNER 3,462,292

ELECTRON INDUCED DEPOSITION OF ORGANIC COATINGS Filed Jan. 4, 1966 4 Sheets-Sheet 4 INVENTOR.

RY C Q/ United States Patent 3,462,292 ELECTRON INDUCED DEPOSITION OF ORGANIC COATINGS Allen H. Turner, Ann Arbor, Mich., assignor to Ford Motor Company, Dearborn, Mich., a corporation of Delaware Filed Jan. 4, 1966, Ser. No. 518,695 Int. Cl. B!) 5/02 U.S. Cl. 117-9331 Claims ABSTRACT OF THE DISCLOSURE A method of electrostatically coating an object which comprises establishing an electric field between the object and a spraying device, spraying charged particles of coating material into said field toward the object and projecting an electron beam into the path of the charged particles to induce a further electrical charge on the particles. Where the coating material is'polymerisable or crosslinkable, for example is vinylically unsaturated, the electron beam may also polymerise or crosslink the coating material.

This invention relates to the art of coating. In particular, this invention is concerned with controlling the pattern of a spray of finely divided particles of a liquid coating material to prevent loss of coating material through overspray of the object to be coated. In general the method of this invention involves projecting discrete particles of paint or other coating material into a contact zone within an electric field extending between a first electrode and said object with said object constituting a positive electrode forming one terminus of said field, an electron beam is transmitted into said contact zone, said particles acquire a negative electrical charge'in relation to said object and are electrostatically deposited upon said object.

In accordance with this invention the spray is subjected to an electron beam the electrons of which have average electrical potential in excess of about 50,000 electron volts, advantageously in the range of about 100,000 to about 500,000 electron volts or higher. This method of electrostatic deposition has the added advantage of being adaptable to the deposition of metallic and Water based paints where atomization thereof is effected by mechanical means.

The terms highenergy electrons and polymerization effecting electrons as employed herein refer to electrons having energy at least sufiicient to ionize organic compounds or break chemical bonds, e.g. electrons having energy in excess of about 50,000 electron volts, advantageously in the range of about 100,000 to about 500,000 electron volts or higher, and preferably in the range of about 175,000 to about 375,000 electron volts.

A high-energy electron source may be provided by accelerating electrons to high energy in an evacuated tube, and permitting the high-energy electrons to issue from the tube through an electron window therein, ordinarily a thin metal sheet.

A variety of devices are disclosed in the literature which are adapted to transmit therefrom an electron beam suitable for use in the method of this invention. See, for example, the following U.S. Patents: Hughes et al., 3,132,- 966; Lawton 3,097,960; Schmitz et al. 2,921,006; Robinson 2,680,814 and 2,602,751; and Westendorf 2,144,518.

Conventionally, the housing of the emitter, or a chamber forming member therein in which both the cathode.

and anode elements are positioned, is substantially gasevacuated by conventional conduit and pumping means, e.g. to an air pressure as low as about 10- mm. Hg, commonly about 2.5 10 to about 5X10 mm. Hg. One or more cathodes, e.g. tungsten wire filaments, are positioned within the evacuated housing and electrically connected to the negative terminal of a direct-current, high-voltage, electrical power source. The metal window conventionally comprises at least a part of the positive electrode. Ordinarily, the window is in contact with a suitable support of conductive material such as a tubular member spaced apart from the cathode and extending from in front of the cathode to the window so as to provide an unimpeded path for electrons to pass from the cathode to the Window. A current suflicient to cause electron emission from the cathodes is provided by means hereinafter discussed. The emitted electrons are accelerated through the vacuum chamber to the electron window and pass through the thin metal window.

In one such device, electrons are accelerated as a narrow beam within the evacuated tube, and then a rapid scanning movement is imparted to the electron beam before it passes through the electron window and issues from the tube. In another such device, an electron beam is focused into sheet form with the tube by a system of cylindrical electron optics. Where precise focusing is not essential, the electron-emitting cathode or cathodes may simply be partially enclosed in a suitable housing within the tube which restricts and directs the electron beam to the electron window.

This invention will be more fully understood by referring to the accompanying drawings wherein:

FIGURE 1 is a semidiagrammatic view of apparatus arranged for carrying out one embodiment of this invention;

FIGURE 2 illustrates one embodiment of an electron accelerator such as those shown in FIGURE 1 with a portion of the outer housing removed to reveal a semidiagrarnmatic view of the components thereof;

FIGURE 3 is a schematic view of one embodiment of electrical circuitry which may be employed in the operation of the electron accelerator shown in FIGURE 2;

FIGURE 4 is a schematic view of the filament transformer shown in FIGURE 3;

FIGURE 5 is a partially cut-away view of another embodiment of apparatus for use in carrying out the method of this invention;

FIGURE 6 is a front view of the electron accelerator shown in FIGURE 5;

FIGURE 7 is an end view of the filament-containing housing of FIGURE 5 with housing cap 227 pulled away from housing 225;

FIGURE 8 is a view taken along line 8-8 of FIGURE 5; and

FIGURE 9 is a semidiagrammatic view ofapparatus arranged for carrying out another embodiment of this invention.

Referring now to FIGURE 1, a metal workpiece 11 is shown suspended by a hanger 13 from a conventional conveyor track 15. Conveyor track 15, hanger 13 and workpiece 11 are in electrical communication with ground. Hanger '13 is being moved along conveyor track 15 by suitable conveyor drive means, e.g. chain, not shown.

Positioned in the front of workpiece 11 is a vertically and circumferentially adjustable support means 17 upon.

which is mounted housing 19. To the front of and supported by housing 19 is an adjustable support rack 21 of paint 39 in the direction of metal workpiece 11 which is passing in a plane perpendicular to the axis of the spray pattern. The paint spraying device 37 can be one of the devices heretofore disclosed in the art for use in spray painting and capable of atomizing and dispersing a liquid coating material as relatively small discrete particles in or from which the paint feed is atomized and dispersed by compressed air, hydrostatic pressure, air pressure and electrostatic forces, hydrostatic pressure and electrostatic forces, centrifugal forces and electrostatic forces, or impact means sometimes termed acoustic type atomizers wherein the paint stream contacts an object vibrating at high frequency. The vibrating unit may have either the fiat form of a conventional reed-like vibrator or that of one or a plurality of needlelike members and may be employed either with or without supplementary electrostatic forces provided by direct or induced electrical charge upon the atomizer or with one or more electrodes spaced apart therefrom so as to provide an electrostatic field intermediate the atomizer and the workpiece.

An exemplary but by no means exhaustive list of spray devices that may be used in the method of this invention includes conventional air atomizing spray guns, e.g. De- Vilbiss models P.I.G.A. 5401 and 502 or Binks models 62 and 62] adapted for automatic and/or non-manual triggering, and spray devices disclosed in US. Patents Nos. 2,247,000 Popoif; 2,491,889 Bennett; 2,754,228 Bede; 2,766,064 Schweitzer; 2,782,074 Sedlacsik; 2,826,541 Sedlacsik; 2,881,092 Sedlacsik; 2,893,894 Ransburg; 3,048,498 Juvinall et al.; 3,163,360 McNinch et al.; 3,169,882 Juvinall et al.; and 3,169,883 Juvinall et al. It will be understood by those skilled in the art that in those embodiments where the spray system includes the employment of an electrostatic field for atomization, dispersion and/or deposition the instant method provides supplementary and/or complementary influence in controlling the spray pattern, e.g. by increasing or modifying spray wrap-around, and that the positioning of the electron accelerators will be such as to assist in the coating process without adversely interfering with the existing electrostatic field.

Insulated conduit 41 is in operative connection with spray device 37 within housing 19 and provides conduit and/or housing means for all necessary connections to the spray device 37, e.g. paint supply means, solvent supply means, cables for high voltage-low current electrical transmission, etc.

Electron beams indicated by numerals 43 and 43-1 from electron acceleration units 27 and 29 are shown impinging upon spray 39 intermediate spray device 27 and workpiece 11.

Housing 19 also contains conventional control systems for triggering paint flow, etc., in accordance with movement of the workpieces through the coating zone. Although not here shown, it will be understood by those skilled in the art that spray device 37 and electron acceleration units 27 and 29 may be operatively connected with conventional reciprocation means for vertical reciprocation where the shape of the workpiece indicates advantage in reciprocation. Object 45 partially shown to the right of workpiece 11 represents an unpainted or prime coated workpiece approaching the coating zone. Object 47 on the left of workpiece 11 represents a coated object that has been painted in accordance with the method of this invention.

In FIGURE 2, one embodiment of electron acceleration unit 29 is shown with a portion of the unit housing 51 removed to reveal the interior components thereof.

Positioned within housing 51 is a glass tubular member 55 from which gas has been substantially evacuated, e.g. to a pressure as low as about 10* mm. Hg, commonly about 2.5 1O to about 5 10 mm. Hg, by pumping means not shown to form a vacuum chamber 57. Extending into and in sealed relationship with tubular member 55 is a hollow metal anode 59. Anode 59 has an aperture 61 closed by an electron window 63 which is held in gas tight relationship with housing 51 by window support 51 form a chamber 67 which conventionally is filled with oil. Also extending inside tubular member 55 is a cathode assembly 71 which includes a filament shield 73 having an aperture 75 communicating with the interior of tubular member 55 and a filament 77 in electrical connnection with a high voltage source, not shown in this figure, via electrical circuitry extending through conduit 79. Aperture 75 provides a straight, uninterrupted passageway between filament 77 and window 63. One embodiment of circuitry for the accelerator of FIGURE 2 is schematically illustrated in FIGURES 3 and 4.

Referring now to FIGURES 3 and 4, there is shown in the upper left of the view a high voltage electrical power supply source 83 in electrical connection with a voltage divider 89 via conductor 87 and conductor which is also in electrical connection with ground. Conductor 91 provides electrical connection between voltage divider 89 and housing 51 which in turn is in electrical connection with anode 59 and is connected with voltage divider 89 in a manner such as to provide tubular member 55 and anode 59 with a voltage V Conductor 93 provides electrical connection between voltage divider 89 and filament 77 within filament transformer 95 and provides a voltage V to filament 77. V and V represent negative potentials with respect to ground. V is more negative with respect to ground than is V Filament 77 is a continuous filament and includes a coil 77-1. Conductor 97 includes a coil 97-1. Conductor 99 is electrically connected to the opposite end of coil 97-1. Conductors 97 and 99 represent leads from a low voltage power source, not shown, e.g. volts A.C., and a current is passed therethrough and through coil 97-1 which induces a current in filament 77 sufficient to provide electron emission from filament 77. Coils '77-1 and 97-1 are provided with suitable insulation, represented by insulator 100, for the operating voltage required. The voltage ditference between V and V provides the acceleration potential for electrons emitted from filament 77. Conductor 91 is also in electrical connection with conductor 101. The spray device 37 can be placed in electrical connection with conductor 101, housing 51 and conductor 91 via switch 103. Switch 103 can be reversed so as to place spray device in electrical connection with grounded conductor 105.

Referring now to FIGURES 5 to 8 inclusive, there is here shown another embodiment of apparatus for carrying out the method of this invention. In this embodiment the electron accelerator 201 has a circular exterior housing 203. Housing 203 may be viewed as a continuous tubular structure forming an axial opening or passageway 203-1. Circumferentially' positioned around one terminus of passageway 203-1 are positioned a plurality of electron windows 205 through which a plurality of electron beams are transmitted and employed as hereinafter described.

Opposite the electron transmitting face of the accelerator are shown insulated conductors 207 and 209 which are in electrical connection with a high-voltage power source, not shown. Conductors 207 and 209 pass through accelerator supports 211 and 213 respectively and thence through insulator supports 215 and 217 respectively wherein the exterior insulation on conductors 207 and 209 terminates.

Conductor 207 emerges from insulator support 215 and passes through locating means 221.

Conductor 209 emerges from insulator support 217, passes through insulator 223, the rear wall of filament housing 225 and insulator 233. One or both of insulators 223 and 233 extend through housing 225 encasing conductor 209 and insulating 209 from housing 225. Housing 225 has a removable housing cap 227. Together they form filament-containing chamber 229. Housing cap 227 is provided with a plurality of openings 231 which are of the same number as and aligned with electron windows 205 in housing 203. Conductor 209 is in electrical connection with conductor bracket 235 which is secured to conductor 209 by fastener means 239. Conductor bracket 235 is in electrical connection with and secured to conductor bolt 241 which is in electrical connection with bus bar 243 and insulated from bus bar 249, shown in FIG- URE 8 by suitable insulator means not shown Between and in electrical contact with bus bar 243 and bus bar 249 are a plurality of tungsten wire filaments 247. Filaments 247 are aligned with openings 231 in filament housing cap 227 and electron windows 205 in housing 203. Bus bar 249 is in electrical connection with conductor bolt 251 which is insulated from bus bar 243 by suitable insulation means not shown. Conductor bolt 251 is secured to and in electrical connection with conductor bracket 253. This terminus of conductor 207 passes through locating means 255 and a housing 225. A slight difference in electrical potential, e.g. 2-5 volts is maintained between conductors 207 and 209 to cause emission providing current through filaments 247. The interior of housing 203 forms a vacuum chamber 261 as in the previously described embodiment. Electron windows 205, aluminum alloyed with a minor amount of copper and about 0.001 inch in thickness, are attached by suitable means to housing 203. Windows 205 and housing 203 are operatively maintained at a potential positive with respective to cathode elements 247, e.g. which are above about 50,000 electron volts negative with respect to ground.

A spray device 137 is shown extending partially through passageway 203-1 and projecting a paint spray 139 toward a workpiece 111. Workpiece 111 is at ground potential. Electron beams 143 and 143-1 are shown emanating from electron windows 205 and impinging upon the paint spray 139 as in FIGURE 1. Spray device 137 is one of the spray devices previously described in connection with FIGURE 1. It is within the scope of this invention to have the coating fluid to emerge from the spray device at a position to the rear of the electron windows as illustrated in FIGURE 5, even with the windows, or forward of the windows as best accommodates the individual installation and usage.

Referring now to FIGURE 9, another embodiment of this invention is shown wherein the electron accelerators are positioned to the side and rear of a workpiece so as to impinge upon a paint spray directed to such workpiece from the front. In this view a workpiece 411 is shown suspended by a hanger 413 from a conveyor track 415. Conveyor track 415, hanger 413 and workpiece 411 are in electrical connection with ground. Hanger 413 is being moved along conveyor track 415 by suitable conveyor drive means, not shown. Positioned in front of workpiece 411 is a housing 419. A paint spraying device 437 is shown extending from the front of housing 419 and emittng a finely divided spray of paint 439 in the direction of workpiece 411 which is passing in a plane perpendicular to the axis of the spray pattern. Housing means 419 is provided with electrical inlet means 435 for connection with an electrical power source where spray device 437 is electrically charged. Conduit 441 is in operative connection with spray device 437 and provides means for supplying paint, solvent, etc., as required. Behind the workpiece 411 is positioned adjustable support rack 421 having pivotable arms 423 and 425 which respectively support electron acceleration units 427 and 429. Units 427 and 429 are in electrical connection with a highvoltage power source, not shown, via insulated cables 431 and 433, respectively. The electron beams indicated at 443 and 443-1 impinge upon paint spray 4.39 as it approaches the workpiece 411 and may be adjusted to impinge upon overspray passing or starting to pass the workpiece.

It is within the scope of this embodiment to have adjustable support rack 421 movable upon track means and Cir operatively connected with the conventional power and triggering means. In this mode of operation workpiece 411 moving along the route of its conveyor moves into optimum position with relation to electron acceleration units 427 and 429, i.e. where the respective electron beams are directed so as to contact the spray approaching and/or attempting to pass the forward and trailing edges of the workpiece 411, the triggering means which may be activated by workpiece movement on the line activates power means which are capable of moving support rack 421 along its associated track means at essentially the same speed as workpiece 411 moves along its predetermined route, support rack 421 thereupon moves with essentially the same speed as workpiece 411 and in essentially the same direction so as to maintain such optimum position while workpiece 411 is receiving paint spray. Upon moving a predetermined distance which may be fixed by a conventional switch, support rack 421 is reversed and returned to its point of origin by the same or different power means in time to repeat the process upon the arrival of the next workpiece into the coating zone.

In other embodiments, the exterior of the accelerator may be at ground potential with a separate electrode spaced apart therefrom and providing the negative terminus of the electric field in which the coating particles are charged by electron bombardment from the accelerator, or in lieu of such separate negative electrode, the workpiece may be positively charged with respect to ground.

A current of at least about one milliampere is advantageous.

In addition to the primary function of the instant invention, i.e. to modify and shape the spray pattern for maximum utilization of paint sprayed through the unique accelerator positioning hereinbefore disclosed, the method can be utilized to effect polymerization of special coating materials simultaneously with electrostatic deposition of the same. To achieve this dual function in accordance with this invention, the accelerator is positioned so that at least a major portion of the electrons of the electron beam or beams emanating from the accelerator pass through the spray pattern to the surface of the workpiece receiving the deposit and reach such surface with average energy within the hereinbefore recited definition of polymerization-eifecting electrons. Voltage and distance are coadjusted to provide this relationship. The coating material for this embodiment requires suitable unsaturation and preferably comprises a resin having about 0.5 to about 3 alpha-beta olefinically unsaturated units per 1,000 units molecular weight which may be employed in combination with vinyl monomers.

It will be understood that the method of this invention may also be used for the electrostatic deposition of suitable finely divided solids.

I claim:

1. In a method for depositing a coating material upon an electrically-conductive object which comprises creating an electric field extending between an electrically charged first electrode and said object which forms one terminus of said field and constitutes a positive electrode relative to said first electrode, propelling electrically charged discrete particles of said coating material from said first electrode into a contact zone within said field, forming a spray pattern of said particles within said field and electrostatically depositing at least a portion of said spray upon said object, the improvement which comprises pro viding electron emission means in position to project an electron beam into said contact zone and transmitting a beam of electrons from said electron emission means into said contact zone and into contact with particles of said coating material in said spray pattern, said electrons upon contacting said particles within said contact zone having average electrical energy sufiicient to induce an additional electrical charge upon at least a substantial number of said particles.

2. The method of claim 1 wherein the axis of said spray pattern is directed horizontally toward said object.

3. The method of claim 1 wherein said spray pattern is initiated from a location for remote from said object than the location at which said electron beam is transmitted from said electron emission means and enters the atmosphere.

4. The method of claim 1 wherein said spray pattern is initiated from a location less remote from said object than the location at which said electron beam is transmitted from said electron emission means and enters the atmosphere.

5. The method of claim 1 wherein said spray pattern is initiated at a distance from said object substantially equal to the average distance between said object and the location at which said electron beam is transmitted from said electron emission means and enters the atmosphere.

6. The method of claim 1 wherein said object is at ground potential.

7. The method of claim 1 wherein the average electrical energy of electrons .in said beam is in the range of about 100,000 to about 500,000 electron volts.

8. In a method for depositing an organic coating material upon an electrically-conductive object which comprises creating an electric field extending between an electrically charged first electrode and said object which forms one terminus of said field and constitutes a positive electrode relative to said first electrode, propelling electrically charged discrete particles of said coating material from said first electrode into a contact zone within said field, forming a spray pattern of said particles within said field and electrostatically depositing at least a portion of said spray upon said object, the improvement which comprises providing electron emission means in position to project electron beams into said contact zone so as to contact at least two opposite sides of said spray pattern and transmitting beams of electrons from said electron emission means into said contact zone and into contact with at least two opposite sides of said spray pattern before the contacted particles reach said object, said electrons upon contacting said particles within said contact zone having average electrical potential sufiicient to induce an additional electrical charge upon at least a substantial number of said particles.

9. The method of claim 8 wherein said spray pattern is contacted substantially circumferentially by a plurality of said electron beams.

10. The method of claim 8 wherein said object moves along a predetermined path intersecting the projected axis of said spray pattern and said electron emission means moves in a predetermined path and maintains essentially the same relative position with respect to said object while said object is moving through said spray pattern.

11. The method of claim 8 wherein said object is stationary during said deposition and said spray pattern moves in a predetermined path.

12. The method of claim 8 wherein said electrons upon contacting said particles of said spray pattern have an average electrical energy in the range of about 100,000 to about 500,000 electron vlots.

13. In a method for depositing a radiation-polymerizable, organic, coating material upon an electrically-conductive object which comprises creating an electric field extending between an electrically charged first electrode and said object which forms one terminus of said field and constitutes a positive electrode relative to said first electrode, propelling electrically charged discrete particles of said coating material from said first electrode into a contact zone within said field, forming a spray pattern of said particles within said field and electrostatically depositing at least a portion of said spray upon said object, the improvement which comprises providing electron emission means in position to project electron beams into said contact zone so as to contact at least two opposite sides of said spray pattern and transmitting beams of electrons from said electron emission means into said contact zone, into contact with at least two opposite sides of said spray pattern before the contacted particles reach said object and into contact with the area of resultant deposit with polymerization-effecting electrons while said particles are being deposited on said object, said electrons upon contacting said particles within said contact zone having average electrical potential sufiicient to induce an additional electrical charge upon at least a substantial number of said particles.

14. The method of claim 13 wherein said electrons upon contacting said particles in said contact zone have an average electrical energy of above about 50,000 electron volts.

15. The method of claim 13 wherein said electrons upon contacting said resultant deposit have an average electrical energy in the range of about 100,000 to about 500,000 electron volts.

Da Silva et al., Formation of Polymer Films by Low Energy Electron Radiation, IBM Technical Disclosure Bulletin, vol. 7, No. 9, February, 1965, p. 737.

ALFRED L. LEAVITT, Primary Examiner I. H. NEWSOME, Assistant Examiner U.S. Cl. X.R. 117-93.4 

